So far there have been three principle types of short takeoff and landing modes:
One is the externally blown flap mode. FIGS. 5 and 6 depict an STOL aircraft which employs this mode. According to this mode, a high lifting capability is obtained by directing gas 8 discharged from a jet engine 15 against flaps 17 connected to the trailing edge of a wing 18. This mode provides a lift coefficient of 6.0. The STOL aircraft of FIGS. 5 and 6 is YC-15 developed in the United States. This mode will hereinafter be referred to as an "EBF mode".
Another is the rearward-stream deflecting mode. FIGS. 7 and 8 depict an STOL aircraft which employs this mode. According to this mode, a high lifting capability is obtained by steeply deflecting an airstream behind a propeller 21 downwardly by a flap 22 connected to the trailing edge of a wing 24. This mode provides a lift coefficient of 5.7 to 6.5. The STOL aircraft of FIGS. 7 and 8 is PS-1 developed in Japan.
The other is the upper surface blowing mode. FIG. 9 depicts a STOL aircraft which employs this mode. For this mode, an exhaust duct 27 for a turbofan engine 26 is located on the upper surface of a wing 25. According to this mode, gas 28 discharged from the turbofan engine 26 flows along the upper surface of the wing 25 and along the upper surfaces of flaps 29 connected to the trailing edge of the wing 25. This mode provides a lift coefficient of 6.0. The STOL aircraft of FIG. 9 is YC-14 developed in the United States. This mode will hereinafter be referred to as a "USB mode".
However, since all the foregoing three conventional STOL modes require the provision of high lift systems, the foregoing STOL aircraft are inferior in their payload-carrying capacities to a conventional takeoff and landing aircraft (CTOL aircraft).
Hence, the conventional STOL aircraft are not of a practical nature.
The rearward-stream deflecting mode of FIGS. 7 and 8 is not suitable for a high-speed flying for the following reason: The airstream behind the propeller 21, accelerated by the propeller 21, collides with the leading edge of the wing 24 (FIG. 8). The speed at which the airstream behind the propeller 21 flows is approximately 1.8 times the flying speed of the aircraft when the airstream is flowing immediately behind the propeller 21. Therefore, if the propeller 21 is located further away from the leading edge of the wing 24, the airstream behind the propeller 21 collides with the leading edge of the wing 24 at a speed which is approximately mach 0.15 higher than the flying speed of the aircraft. Thus, the rearward-stream deflecting mode is not suitable for an aircraft which is required to fly at a speed obtainable immediately before the wing produces a shock wave, that is, at approximately mach 0.8. The maximum economical flying speed of an aircraft is delimited by the shock wave which is produced by the wing. FIG. 13 depicts increases in the coefficient C.sub.D of the drag produced by air, in relation to the flying speed M (mach). FIG. 13 shows that, no matter what the ratio of the profile thickness to the chord length is, the aircraft is subjected to a large drag when the aircraft is flying with the range from mach 0.80 to mach 0.85. Thus, the rearward-stream deflecting mode only provides a flying speed of mach 0.65 to mach 0.70.
With the USB mode, the gas discharged from the turbofan engine 26 first turns upwardly for some 8 degrees and then turn downwardly for some 8 degrees and then flows along the upper surface of the wing 25. Hence, an exhaust nozzle 32 of a core engine 31 is specially formed. That is, the exhaust nozzle 32 is turned upwardly for some 8 degrees. Also, an exhaust duct 27 is bent accordingly and, hence, part of the energy is lost. In addition, the gas discharged from the exhaust duct 27 causes a friction on the upper surface of the wing 25, thus reducing the thrust of the engine.
Thus, neither of the rearward-stream deflecting mode and the USB mose is capable of effective function.